1. Field of the Invention
This invention is concerned with nuclear reactor damage prevention, more particularly with preventing occurrence of a coolant void at the top of the nuclear fuel assembly from sudden loss of reactor coolant system (RCS) inventory level caused by nozzle dam removal at the completion of a maintenance cycle.
2. Description of the Prior Art
There exists the possibility of a sudden, uncontrolled, substantial drop in reactor coolant system inventory level when a nozzle dam is removed, more specifically at the moment when the seal between the nozzle dam and the nozzle is broken, after the nozzle dam has been in place for sufficient time to perform an average maintenance cycle of several weeks or longer.
If the drop is severe enough, the water will fall below the level of the reactor hot side opening to the residual heat removal system (RHR), starving the system and stalling the RHR pump. This can result in overheating and water not reaching the exposed top of the core. This is a potentially hazardous condition which must be prevented.
A phenomena that is likely to be associated with this potential condition is an uncontrolled upward bounding of the nozzle dam, which can be hazardous to personnel unbolting the dam from the nozzle. This also must be prevented.
Indications of the phenomena and lowered RCS inventory level were observed for example at the following instances, when nozzle dams were being removed after the usual industry procedure of lowering the water level to mid loop from being at the refueling level for some extended period. The following data is based on recollections by the technicians and on-site management personnel. No coordinated data collection, however, was made at the time to specifically record changes in RCS inventory levels during nozzle dam removal.
In the summer of 1990, and spring of 1991, at the Catawba plant, Unit 2, 4-loop system with common drain on steam generator. The cold leg dam bounded. Although one of the incidents was recorded on video tape, the RCS inventory levels were not recorded.
In the Spring of 1992, at the Callaway plant, 4-loop system, common drain on steam generator. The dams bounded in the 4th and 8th cold legs. The control room reported a substantial drop in the Reactor coolant system inventory level after removal of the 4th and 8th cold legs.
In the Fall of 1992, at Diablo Canyon plant 4-loop, individual drain on steam generator system, a cold leg dam bounded.
In present industry practice, after primary water is drained below the steam generator bowl, down to mid loop, and before it is restored to refueling level above the steam generator bowl, every precaution is taken to hermetically seal off the steam generator bowl from the hot and cold leg nozzles by a nozzle dam for each nozzle, and by a drain plug for each individual drain conduit between the bowl, also termed "channel head", and nozzle which bypasses the dam.
Once the water is restored to the refueling level, leakage may occur around the seal of some nozzle dams. The typical procedure, as described in U.S. Pat. No. 4,959,192, Trundle et al, patented Sep. 25, 1990, is to monitor for leakage into the bowl, the leakage being acceptable provided a bowl drain pump can keep up with the leakage.
Many nozzle dams, especially including ones with inflated seals such as the BUSI Nozzle Dam, available from Brand Utility Services Inc., and generally described in U.S. Pat. No. 4,957,215, Evans et al., patented Sep. 18, 1990, are designed to have no leakage across the seal barrier between the nozzle and the bowl interior once the dam is bolted in place. Having a combination of passive and inflated seals, the BUSI Nozzle Dam usually seals against cross leakage even without the inflatable seals being inflated.
Inflatable seals are also described, for example, in U.S. Pat. No. 4,482,076, Wentzell, patented Nov. 13, 1984, and in U.S. Pat. No. 4,690,172, Everett, patented Sep. 1, 1987.
In removing a nozzle dam, it is standard in the industry, as described in U.S. Pat. No. 4,959,192, Trundle, to remove the nozzle dam after the reactor coolant system has been drained to mid loop. Trundle suggests that it is preferable to remove the hot leg dam prior to the cold leg dam. He also suggests that in a system having an individual drain conduit to remove the drain plug after draining down below the nozzle dam in order to confirm that the loop is adequately drained, that is, drained below the level of the nozzle as indicated by an absence of water leaking up from the drain conduit, before removing the nozzle.
He also suggests to allow several minutes to elapse between removal of the drain plug after the water level is drained below the nozzle dam and attempting removal of the nozzle dam when working in the cold leg, in order to allow any low pressure caused by draining the loop to dissipate.
Although the above steps and precautions which are characteristic in the field are useful and continue to be advisable in installing and removing a nozzle dam and drain plug, they do not prevent chance of the aforedescribed bounding or of an uncontrolled drop in reactor coolant system, and may bring on the conditions leading to the occurrences.
The present invention is designed to prevent the above described phenomena of nozzle dam bounding and sudden excessive drop in reactor coolant system inventory level when a nozzle dam is removed from the nozzle.